1. Field of Invention
The present invention relates to a method for making an inexpensive poly-silicon solar cell and, more particularly, to a method for making a titanium-based compound film of a poly-silicon solar cell and, more particularly, to a method for making a titanium-based compound film on a ceramic substrate in a high-temperature process based on chemical vapor deposition (“CVD”).
2. Related Prior Art
Most silicon-based solar cells are made in low-temperature processes based on plasma-enhanced chemical vapor deposition (“PECVD”). An amorphous-silicon or microcrystalline-silicon film is coated on a glass, aluminum, silicon, stainless steel or plastic substrate. A back contact is made of aluminum, gold, silver or transparent conductive oxide such as indium-tin oxide (“ITO”) and zinc oxide.
The primary advantage of the low-temperature processes is the wide variety of materials that can be used to make the substrates. However, they suffer drawbacks such as defective silicon films, low photoelectrical conversion efficiencies and low light-soaking stability.
In the PECVD, while coating the microcrystalline silicon film, a silicon material is highly diluted in hydrogen. For example, [H2]/[SiH4]>15. That is, the concentration or flow rate of H2 is more than 15 times as high as that of SiH4. The problems with the PECVD are a low growth rate of the film, a long process and a high cost.
Regarding the making of the poly-silicon solar cells, there are various techniques such as solid phase crystallization (“SPC”) and aluminum-induced crystallization (“AIC”). The SPC is based on the PECVD. An amorphous silicon film is deposited, intensively heated and annealed at a high temperature. Thus, a poly-silicon film with a grain size of 1 to 2 micrometers is made.
In the AIC as shown in FIGS. 14 through 18, an aluminum film 92 is coated on a substrate 91. An amorphous silicon film 93 is coated on the aluminum film 92 based on the PECVD and annealed at a temperature of about 575 degrees Celsius for a long time to form a seeding layer 94. Then, it is subjected to an epitaxial process such as the PECVD or an electron cyclotron resonance chemical deposition (“ECR-CVD”) to make a poly-silicon film 95. The AIC however involves many steps and takes a long time. The resultant grain size is about 0.1 to 10 micrometers.
As discussed above, regarding the conventional methods for making poly-silicon film solar cells in the low-temperature processes based on the PECVD, there are many defects in the silicon films, the photoelectrical conversion efficiencies are low, the light soaking stabilities low, the growth rates of the films low, the processes long, and the costs high. Concerning the method for making poly-silicon film solar cells based on the AIC, the processes are long for including many steps and therefore expensive.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.